1. Field of the Invention
The various embodiments of the present invention generally relate to methods of processing nickel-titanium alloys. More particularly, certain embodiments of the present invention relate to thermally processing nickel-titanium alloys to predictably adjust the austenite transformation temperature and/or transformation temperature range of the alloy.
2. Description of Related Art
Equiatomic and near-equiatomic nickel-titanium alloys are known to possess both “shape memory” and “superelastic” properties. More specifically, these alloys, which are commonly referred to as “Nitinol” alloys, are known to undergo a martensitic transformation from a parent phase (commonly referred to as the austenite phase) to at least one martensite phase on cooling to a temperature below the martensite start (or “Ms”) temperature of the alloy. This transformation is complete on cooling to the martensite finish (or “Mf”) temperature of the alloy. Further, the transformation is reversible when the material is heated to a temperature above its austenite finish (or “Af”) temperature. This reversible martensitic transformation gives rise to the shape memory properties of the alloy. For example, a nickel-titanium alloy can be formed into a first shape while in the austenite phase (i.e., above the austenite finish temperature, or Af, of the alloy), and subsequently cooled to a temperature below the Mf and formed into a second shape. As long as the material remains below the As (i.e., the temperature at which the transition to austenite begins or the austenite start temperature) of the alloy, the alloy will retain the second shape. However, if the alloy is heated to a temperature above the Af the alloy will revert back to the first shape.
The transformation between the austenite and martensite phases also gives rise to the “superelastic” properties of nickel-titanium alloys. When a nickel-titanium alloy is strained at a temperature above Ms, the alloy can undergo a strain-induced transformation from the austenite phase to the martensite phase. This transformation, combined with the ability of the martensite phase to deform by movement of twinned boundaries without the generation of dislocations, permits the nickel-titanium alloy to absorb a large amount of strain energy by elastic deformation without plastically (i.e., permanently) deforming. When the strain is removed, the alloy is able to almost fully revert back to its unstrained condition.
The ability to make commercial use of the unique properties of nickel-titanium alloys, and other shape memory alloys, is to a great extent dependent upon the temperatures at which these transformations occur, i.e, the As and Af, and Ms and Mf of the alloy, as well as the range of temperatures over which these transformations occur. However, in binary nickel-titanium alloy systems, it has been observed that the transformation temperatures of the alloy are highly dependent on composition. That is, for example, it has been observed that the Ms temperature of a nickel-titanium alloy can change more than 100K for a 1 atomic percent change in composition of the alloy. See K. Otsuka and T. Kakeshia, “Science and Technology of Shape-Memory Alloys: New Developments,” MRS Bulletin, February 2002, at pages 91–100.
Further, as will be appreciated by those skilled in the art, the tight compositional control of nickel-titanium alloys necessary to achieve predictable transformation temperatures is extremely difficult to achieve. For example, in order to achieve a desired transformation temperature in a typical nickel-titanium process, after a nickel-titanium ingot or billet is cast, the transformation temperature of the ingot must be measured. If the transformation temperature is not the desired transformation temperature, the composition of the ingot must be adjusted by remelting and alloying the ingot. Further, if the ingot is compositionally segregated, which may occur for example during solidification, the transformation temperature of several regions across the ingot must be measured and the transformation temperature in each region must be adjusted. This process must be repeated until the desired transformation temperature is achieved. As will be appreciated by those skilled in the art, such methods of controlling transformation temperature by controlling composition are both time consuming and expensive. As used herein, the term “transformation temperature(s)” refers generally to any of the transformation temperatures discussed above; whereas the term “austenite transformation temperature(s)” refers to at least one of the austenite start (As) or austenite finish (Af) temperatures of the alloy, unless specifically noted.
Methods of generally increasing or decreasing the transformation temperatures of nickel-titanium alloys using thermal processes are known in the art. For example, U.S. Pat. No. 5,882,444 to Flomenblit et al. discloses a memorizing treatment for a two-way shape memory alloy, which involves forming a nickel-titanium alloy into a shape to be assumed in the austenitic phase, and then polygonizing the alloy by heating at 450° C. to 550° C. for 0.5 to 2.0 hours, solution treating the alloy at 600° C. to 800° C. for 2 to 50 minutes, and finally aging at about 350° C. to 500° C. for about 0 to 2.5 hours. According to Flomenblit et al., after this treatment, the alloy should have an Af ranging from 10° C.–60° C. and a transformation temperature range (i.e., Af–As) of 1° C. to 5° C. Thereafter, the Af of the alloy may be increased by aging the alloy at a temperature of about 350° C. to 500° C. Alternatively, the alloy may be solution treated at a temperature of about 510° C. to 800° C. to decrease the Af of the alloy. See Flomenblit et al. at col. 3, lines 47–53.
U.S. Pat. No. 5,843,244 to Pelton et al. discloses a method of treating a component formed from a nickel-titanium alloy to decrease the Af of the alloy by exposing the component to a temperature greater than a temperature to which it is exposed to shape-set the alloy and less than the solvus temperature of the alloy for not more than 10 minutes to reduce the Af of the alloy.
However, there remains a need for an efficient method of predictably controlling the austenite transformation temperatures and/or austenite transformation temperature range of nickel-titanium alloys to achieve a desired austenite transformation temperature and/or austenite transformation temperature range. Further, there remains a need for a method of predictably controlling the austenite transformation temperatures and austenite transformation temperature range of nickel-titanium alloys having varying nickel contents.